From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id B27B4C433FE for ; Thu, 19 May 2022 12:13:47 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id D750C6B0072; Thu, 19 May 2022 08:13:46 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id D24846B0073; Thu, 19 May 2022 08:13:46 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id C3B596B0074; Thu, 19 May 2022 08:13:46 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id B07BE6B0072 for ; Thu, 19 May 2022 08:13:46 -0400 (EDT) Received: from smtpin09.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id 7F520BE5 for ; Thu, 19 May 2022 12:13:46 +0000 (UTC) X-FDA: 79482383652.09.5979A24 Received: from outbound-smtp22.blacknight.com (outbound-smtp22.blacknight.com [81.17.249.190]) by imf24.hostedemail.com (Postfix) with ESMTP id EC43C180005 for ; Thu, 19 May 2022 12:13:34 +0000 (UTC) Received: from mail.blacknight.com (pemlinmail03.blacknight.ie [81.17.254.16]) by outbound-smtp22.blacknight.com (Postfix) with ESMTPS id 50FCBBABFC for ; Thu, 19 May 2022 13:13:44 +0100 (IST) Received: (qmail 28743 invoked from network); 19 May 2022 12:13:44 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[84.203.198.246]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 19 May 2022 12:13:44 -0000 Date: Thu, 19 May 2022 13:13:41 +0100 From: Mel Gorman To: Vlastimil Babka Cc: Andrew Morton , Nicolas Saenz Julienne , Marcelo Tosatti , Michal Hocko , LKML , Linux-MM Subject: Re: [PATCH 4/6] mm/page_alloc: Remove unnecessary page == NULL check in rmqueue Message-ID: <20220519121341.GV3441@techsingularity.net> References: <20220512085043.5234-1-mgorman@techsingularity.net> <20220512085043.5234-5-mgorman@techsingularity.net> <5a0c9b7c-3620-3e0f-7510-d0fc3fa6ceda@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <5a0c9b7c-3620-3e0f-7510-d0fc3fa6ceda@suse.cz> User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: EC43C180005 X-Stat-Signature: imqqmpj73wi3bujw7ayaumufux6qxd4j Authentication-Results: imf24.hostedemail.com; dkim=none; dmarc=none; spf=pass (imf24.hostedemail.com: domain of mgorman@techsingularity.net designates 81.17.249.190 as permitted sender) smtp.mailfrom=mgorman@techsingularity.net X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1652962414-975300 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, May 19, 2022 at 12:57:01PM +0200, Vlastimil Babka wrote: > On 5/12/22 10:50, Mel Gorman wrote: > > The VM_BUG_ON check for a valid page can be avoided with a simple > > change in the flow. The ZONE_BOOSTED_WATERMARK is unlikely in general > > and even more unlikely if the page allocation failed so mark the > > branch unlikely. > > Hm, so that makes a DEBUG_VM config avoid the check. On the other hand, > it puts it on the path returning from rmqueue_pcplist() for all configs, > and that should be the fast path. So unless things further change in the > following patches, it doesn't seem that useful? > You're right -- the fast path ends up with both a if (page) and if (!page) checks. Andrew, can you drop the patch mm-page_alloc-remove-unnecessary-page-==-null-check-in-rmqueue.patch from your tree please? Originally the flow was important when I was writing the patch and later became unnecessary. However, it reminded me of another problem I thought of when writing this and then forgotten to note it in the changelog. If the page allocation fails then ZONE_BOOSTED_WATERMARK should still be tested and cleared before waking kswapd. It could happen if an allocation attempt tried to fallback to another migratetype and still fail to find a suitable page. This is true whether going through the PCP lists or not. So what do you think of me adding this patch to a follow-up series? --8<-- mm/page_alloc: Remove mistaken page == NULL check in rmqueue If a page allocation fails, the ZONE_BOOSTER_WATERMARK should be tested, cleared and kswapd woken whether the allocation attempt was via the PCP or directly via the buddy list. Remove the page == NULL so the ZONE_BOOSTED_WATERMARK bit is checked unconditionally. As it is unlikely that ZONE_BOOSTED_WATERMARK is set, mark the branch accordingly. Signed-off-by: Mel Gorman --- mm/page_alloc.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 1c4c54503a5d..61d5bc2efffe 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3765,12 +3765,10 @@ struct page *rmqueue(struct zone *preferred_zone, page = rmqueue_buddy(preferred_zone, zone, order, alloc_flags, migratetype); - if (unlikely(!page)) - return NULL; out: /* Separate test+clear to avoid unnecessary atomics */ - if (test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags)) { + if (unlikely(test_bit(ZONE_BOOSTED_WATERMARK, &zone->flags))) { clear_bit(ZONE_BOOSTED_WATERMARK, &zone->flags); wakeup_kswapd(zone, 0, 0, zone_idx(zone)); }